The car (2010 4 cylinder) has about 130k miles on it and a while ago a P0420 code was set. I cleared it and all was well. Now 10k miles later the P0420 code has been set again. There are two confusing things.
(1) After warming up the catalytic converter by driving for about 30 mins on the highway I read the following temperatures with an infrared thermometer:
converter input (read directly on the top O2 sensor - accessible from top of engine bay) ~ 390 degF
converter output (read directly on the bottom O2 sensor - also accessible from top of engine bay) ~ 330 degF
A working cat should have a much higher output temperature than input.

(2) Using OBD Fusion to monitor (log) the O2 sensor voltage while driving I only see activity on Bank 1 sensor 2. Bank 1 sensor 1 (which I'd have thought is the cat input O2 sensor) shows constant 0V. But the short and long term fuel trims on Bank 1 do vary with time so perhaps Bank 1 sensor 2 is the cat input O2 sensor since trims cannot be computed if the cat input sensor is not working.

Anyone have any comments? The likelihood of two simultaneous failures (i.e., both the cat output O2 sensor and the cat itself) seems low but I cannot interpret these results any other way. Perhaps I need to take those temperatures again at different spots?

 

You're not getting any misfire codes? Change your o2 sensor and youll be fine. Make sure it's a Denso

 

No other codes besides the P0420 have been set. And I’ve recently confirmed (from a Honda service bulletin) that the cat input (i.e., upstream) sensor is indeed bank 1 sensor 2. So, yes, I’ll replace the downstream O2 sensor.

Thanks

 

The a/f sensor upstream of the cat is bank 1 sensor 1. The a/f sensor sends a different signal compared to old school O2 sensors, so be careful when reading that voltage.

The sensor that is in the cat is bank 1 sensor 2. That is an old school O2 sensor that has the voltage change depending on O2 levels in your exhaust.

The most likely culprit based on your temperature readings is that your converter is not working efficiently. Replacing the sensor in the cat is the gamble some people take, since it is much cheaper to replace compared to the catalytic converter.

 

Here is a good video on explaining the P0420.

 

The first catalytic converter is bolted directly onto the cylinder head (between the engine & firewall). Sensor-1 is #7 in the picture, above the cat. Sensor-2 is #8 which is near the bottom of the cat and probably easier to see from below the car.



There's no sensors at all in the 2nd catalytic converter underneath the middle of the car.

 

Thanks for the comments and diagram. But can anyone make sense of the fact that OBD Fusion reports constant 0V for bank 1 sensor 1 and quickly-varying (0.1V to 0.6V) bank 1 sensor 2 together with varying bank 1 short-term fuel trims? That is, fuel trims are being computed. How could fuel trims be computed if the pre-cat sensor output is a constant 0V?

 

Up through 2002 or so, it was pretty clear that the first O2 sensor was used for fuel trim, and the 2nd sensor was simply to verify the health of the catalyst and had no authority over fuel rate.

Now forward 8 years or so: the 1st catalyst and both sensors are very close to the engine and responsive, and computing power is much greater. I wouldn't be surprised if both sensors are used together somehow.

 

The primary upstream sensor is an a/f sensor, so it does not put out a varying voltage signal like the secondary O2 sensor.

Here is a detailed video on a/f sensors and how they work. The guy can be condescending but knows the basics behind these sensors.

 

I just purchased a 2010 (172K miles) and was told the service engine light was on and the car would not accelerate but then it went away and the SEL went out. Pulled the codes last night. P0420 and P0304; #4 misfire. Cleared the codes and they came right back on subsequent scans. Any recommendations? I experienced the same O2 voltage as OP.